I can only speculate onto why the labels are "Pages" and not "Posts" but I do know that `post` is on the list of reserved post types. The fact is that `post` is *already registered* by default as your normals Posts so you cannot re-register it. Here's a list of [Default Post Types][1] with their respective post type slugs.

What you want to do is actually create your own [Custom Post Type][2]. [Steps to register Custom Post Type][3].


  [1]: https://codex.wordpress.org/Post_Types
  [2]: https://codex.wordpress.org/Post_Types#Custom_Post_Types
  [3]: https://wordpress.stackexchange.com/a/185591/7355